the pigmentary effector system v. the nervous … · o 09 50 pallor complete, o 11 00 noticeably...

295

THE PIGMENTARY EFFECTOR SYSTEM V. THENERVOUS CONTROL OF EXCITEMENT

PALLOR IN REPTILES

BY LANCELOT T. HOGBEN AND LOUIS MIRVISH.

From the Department of Zoology, University of Cape Town.

(Received nth December 1927.)

(With Seven Text-figures.)

1. INTRODUCTION.

THE chromatic responses of the chameleon have excited curiosity from the earliesttimes. They were known to Aristotle, who wrote concerning them: "the change inthe colour of its skin takes place when it is filled with air. It can acquire eithera black colour like that of a crocodile, or ochreous like that of a lizard, or spottedwith black like the panther; for the eyes also change like the rest of the body,and so does the tail1" (Book 11, History of Animals).

Nevertheless, the phenomena of colour change have been studied less ex-tensively in Reptiles than in Fishes and Amphibia. Among those who have in-vestigated the physiology of pigmentary response in the chameleon may be men-tioned Bert, Briicke, Krukenberg and Keller. The only recent investigations oncolour change in Reptiles worthy of note are embodied in the memoirs of Schmidt(1912) on the histology of reptilian pigmentary effectors, and those of Parker (1906)and Redfield (1918) on the behaviour of the Mexican horned "toad" Phrynosoma.Though a variety of pigments occur in the skin of Reptiles, the predominant agentsof colour change in all cases are—as in Fishes and Amphibia—the melanophores.The melanophores of Reptiles are unicellular branched cells, and according to theunanimous testimony of Brucke, Keller, Thilenius, Carlton, Parker and Schmidt,their activity depends on the migration of the pigment along the cell processes.Three categories of external stimuli are known to be effective in the determinationof colour change—light, temperature and mechanical or other nocuous stimuli.As in Amphibia and Fishes, Reptiles respond to warmth by pallor; but the reactionto bright illumination is darkening of the skin, whereas generally speaking—Necturus being a notable exception—the reverse is the case with Amphibia andFishes. Both these reactions are essentially local (vide infra) and it has been heldby many investigators that they depend on the direct excitation of the melano-phores. In the case of nocuous stimulation however locally applied stimuli evokegeneralised pallor, so that a mechanism of co-ordination is evidently involved.

1 CresswelFs translation.

296 LANCELOT T. HOGBEN and Louis MIRVISH

The nature of this mechanism was regarded by earlier workers as nervous.A somewhat different interpretation has been proffered on the other hand byRedfield. The phenomenon of excitement pallor was extensively studied byRedfield in Phrynosoma (1918). As the result of his investigations this workercame to the conclusion that the main factor in distributing the stimulus to chromaticresponse after nocuous stimulation is the liberation of adrenaline into the circu-lation. It had long been known that adrenaline induces contraction of the melano-phores of Vertebrates. And the new evidence adduced by Redfield seemed topoint very strongly to an endocrine control of excitement pallor. The chief pointswhich he records are: (a) failure of animals to respond to electrical stimulation ofthe roof of the mouth after section of the cord at the point between the eighth andthirteenth vertebra, (b) failure of animals to respond in most cases after epi-nephrectomy, (c) failure of local section of nerves to selected areas to interfere withthe responses in any way.

Following as they did upon Cannan's researches into the role of the adrenalsin sympathomimetic accompaniments of excitement in the Mammal, these resultswere fully consonant with what appeared to be the correct explanation of theconsequences of prolonged excitement in warm-blooded Vertebrates. The criti-cisms that have been brought to bear upon the work of Caiman and his collaboratorsby Stewart and his colleagues have re-opened the question; and it is now verydoubtful whether it is possible to define any specific conditions in which increasedliberation of adrenaline from the suprarenal medulla of intact mammals takesplace. Consequently the determination of excitement pallor in Reptiles acquires,as Stewart himself has observed, a new interest in relation to the attempt to in-terpret a physiological, as opposed to pharmacodynamic, role for adrenaline inthe animal body.

Phrynosoma, though displaying the chromatic reaction more noticeably thanmost other common lizards of the North American continent, is by no means themost felicitous type to select for the study of this phenomenon. As is proverbial,the chameleons among Lacertilia afford the most striking display of colour change,and this family is well represented in South Africa. The viviparous speciesChamaeleo pumilus is particularly abundant in the Cape Peninsula, and was theform selected by the writers for investigation. There does not seem to have beenany investigations on the control of colour response in the chameleon, since theintervention of endocrine agencies (cf. Hogben and Winton, 1922-23 and Hogben,1924) in the co-ordination of pigmentary effector activity has been established.

Individual chameleons vary considerably in colour, a fact that had given riseto the popular misconception that a single individual can change from any onecolour to any other. Needless to say any given individual can only change from adefinite light to a definite dark tint. Selected individuals which in the dark con-dition were a deep olive, changing through green to yellow, were employed in theexperiments recorded below. All the individuals were females. It is a curiousfact that of about two hundred animals collected for these experiments, not morethan half a dozen were males.

Excitement Pallor in Reptiles 297

2. THE NORMAL CHROMATIC REACTIONS OF THE CHAMELEON.

As indicated above, the main purpose of the investigation was to determinewhether the phenomenon of excitement pallor provides evidence of conditionsunder which the adrenals discharge their active product into the blood stream inincreased amount. It was necessary however for this purpose to confirm earlierobservations on the normal sequence of pigmentary changes in the species in-vestigated. As regards the influence of light, there is no doubt about the salientfacts which have been established with a few exceptions for reptiles in general,viz. (1) that the exclusion of the organs of vision does not prevent the darkeningwhich accompanies exposure to bright illumination at ordinary temperatures(10-200 C.) (Bert, Keller, Krukenberg); (2) that areas locally illuminated respondlocally by darkening (Bert, Briicke, Keller, Parker, Redfield). The conclusiondrawn from these facts by most investigators has been that reptilian melanophoresrespond directly to light without the intervention of the nervous system, andRedfield (1918) upholds this view with experiments in which peripheral sectionsof the nerve supply of local areas was carried out. Briicke however regarded thephenomenon as dependent on nervous co-ordination, a possibility not to be ex-cluded in view of the evidence brought forward by Parker (1906) to show thatthe skin of Amphibia possesses photo-receptors of a simple kind. Briicke based hisview on the effects of spinal transection. His results have been criticised by Fuchson the grounds that his preparations had not recovered effects of "shock." In thecourse of the present investigations chameleons were kept alive after section ofthe cord or cord and sympathetic chain, for more than a week, and in all caseswhere the region posterior to the point of section failed to respond to stimulationof the mouth, the exclusion of light only resulted in pallor anterior to the point ofsection. Since these animals responded by pallor to electrical stimulation of thecloaca on the side posterior to the point of section, the results of shock may besaid to have been adequately eliminated. Similarly with regard to temperature,exposure to a source of warmth in chameleons in which the cord and chain hadbeen cut at say the fourteenth vertebra resulted in complete pallor anterior to thepoint of section only. These data might be interpreted in the sense defined above,as inferred by Briicke. His interpretation may be put explicitly as follows: (1) thatthe melanophores of the chameleon are normally maintained in a state of "tone"controlled by a centre in the anterior part of the c.N.s.; (2) that light acts reflexlythrough this centre, by releasing them from this state of tone; (3) that warmthreflexly augments the tonic control of the melanophores. On the other hand, thedata could equally well be harmonised with Redfield's view by different assump-tions, namely, that the state of expansion of the melanophores at any given momentis due partly to a more or less continuous state of tone, partly to the direct actionof light in releasing them from this extrinsic control, partly to the effect of warmthin directly influencing them so as to augment cumulatively the tone effect. Onthis view the pallor that normally occurs in darkness would result from the factthat the tonic control is no longer antagonised by light while the failure when


darkened to exhibit pallor in the region posterior to the point of section of the cordwould be due to the fact that the impulses tending to keep the melanophores in astate of sustained contraction no longer reach the area in question. Withoutcutting all dorsal roots of the spinal nerves, a well-nigh impossible operation, thereis no apparent method of distinguishing between the two alternatives experi-mentally. The latter is however the more economical hypothesis.

All the experiments which follow deal with the analysis of excitement pallor.Chameleons do not readily respond to rough handling by pallor; and even electricalstimulation of the skin usually evokes only localised effects. On the other handthe application of a faradic current from an ordinary shocking coil to the roof ofthe mouth or merely mechanical stimulation of the cloaca calls forth a generalisedlightening of tint. Mechanical stimulation of the roof of the mouth failed to evokethe response. The time relations of excitement pallor are worth noting in contrastwith the phenomena of colour change in Amphibia. They are illustrated in thefollowing protocol:

Timeh. m. s.o 00 00 Stimulation of roof of mouth,o 00 30 Stimulation ceased. Noticeable pallor,o 00 50 Pallor complete,o 02 00 Noticeably darker.o 09 00 Darkening complete. Stimulation of roof on mouth,o 09 30 Stimulation ceased,o 09 50 Pallor complete,o 11 00 Noticeably darker.o 19 30 Stimulated roof of mouth. Darkening not quite complete,o 20 00 Stimulation ceased,o 20 05 Complete pallor.

In general it was found that pallor was complete within from half to two minutesfrom the beginning of stimulation. All experiments in relation to this questionwere performed on chameleons kept in bright light at room temperature (18-210 C).The conditions were such as to ensure that the skin remained dark during thecourse of th? operations except when stimulated.

3. THE NERVOUS CONTROL OF EXCITEMENT PALLOR.

The generalised pallor which follows after stimulation of the roof of the mouthor cloaca in the manner indicated implies the intervention of a co-ordinatingmechanism. To ascertain how far nervous and endocrine agencies enter into thisphenomenon, the first question to be investigated was the effect of section of thec.N.s. at different levels; and as there was little difficulty in obtaining largesupplies of chameleons, it was possible to investigate this issue extensively byexperiments involving (1) section of the cord alone at different levels, (2) sectionof the cord and both of the sympathetic chains simultaneously at different levels,(3) section of the cord with unilateral section of the chain, (4) section of the chainalone.

{a) Section of the cord alone. When the cord is cut at any level in the trunkregion, stimulation of the cloaca produces pallor only on the side posterior to the cut


(Text-fig. 3). The two regions as in all the segmental effects hereunder describedare very sharply defined (Text-fig. 2). When the cord is cut at any level anterior to apoint corresponding to the tenth or eleventh vertebra—in extreme cases thecritical point varied individually from behind the ninth to the twelfth in oneinstance—stimulation of the roof of the mouth results in complete pallor on theside anterior to the point of section, while the region behind remains completelydark (see Text-fig. 1, a, b7 c). How striking is the localisation of the response may

a. 5th vertebra c. 10th vertebra

b. 8th vertebra d. 13th vertebra

Fig. i . Effects of stimulation of mouth after section of cord only at different levels.(The shaded area remains dark.)

be inferred from an actual photograph here reproduced (p. 300). Behind thisregion—from the eleventh or twelfth vertebra backwards that is to say—the effectsof section of the cord alone were quite different (Text-fig. 1, d). On stimulationof the roof the mouth in animals so treated, the result obtained was a generalisedpallor affecting the whole body with the exception in nearly all cases of a shortregion at the distal extremity of the tail. These experiences, obtained repeatedlyon a large series of animals, showed conclusively that a nervous agency of somekind is the main factor in the co-ordinating mechanism; but they leave open thepossibility that adrenal secretion plays an adjuvant role in the phenomenon. For,

BJEB'Viv 2O

3<x> LANCELOT T. HOGBEN and Louis MIRVISH

if the nerve supply of the adrenals emerges just in front of the tenth or eleventhvertebra, it is possible that the pallor of the posterior half of the animal when thecord is cut behind this point results from liberation of adrenaline into the circula-tion. This possibility did not seem at the outset a likely one in view of the conse-quence obtained from cloacal stimulation after transection of the cord in the same

Fig. 2. Photograph of chameleon showing pallor anterior to point of sectionat tenth vertebra after stimulation of roof of mouth.

12th vertebra 15 th vertebra

Fig. 3. Effects of stimulation of cloaca after section of cord only in posteriorregion (12th and 15th vertebra).

region of the trunk, and in view of the fact that the generalised pallor resultingfrom stimulation of the mouth after transection at the level of e.g. the thirteenthor fourteenth vertebra was perfectly uniform both as regards its extent and rateof development. As will be seen, the effects of section of the cord and chain simul-taneously provide conclusive evidence for rejecting the view that adrenal secretionhas any significant influence in contributing to excitement pallor. One fact howeveris worthy of mention here. In order to exclude the possibility that adrenalinesecretion may reinforce the effects of continued stimulation, stimulation of the

12th vertebra 14th vertebra

Fig. 4. Effects of stimulation of mouth after section of cord and sympatheticchain of both sides in posterior region.

la

lla lib

Fig. 5. Effects of stimulation of mouth after section of cord and unilateralsection of sympathetic chain:

I a. Left side: chain cut at same level as cord (13th vertebra).I b. Right side: chain intact.II a. Left side: chain intact.II b. Right side: chain cut at same level as cord (12th vertebra).

20-2


mouth or cloaca in cases, where purely segmental responses occurred, was in somecases protracted for a period of ten minutes or even a quarter of an hour.

(b) Section of the cord and chain. When in addition to cutting the cord, thesympathetic chain is cut on both sides at the same level, the result of stimulatingthe mouth is a pallor confined to the region anterior to the point of section, evenwhen the cut is made in the region behind the eleventh vertebra (Text-fig. 4). Intwo experiments in which the cord was cut at the level of the twelfth and thirteenthvertebra, and the chain was cut on both sides at the level of the sixteenth vertebra,pallor extended after stimulation of the roof of the mouth beyond the point oftransection of the cord as far as the level, where the chain was cut. In this casethe chain was cut behind the region where the adrenals are located.

(c) Section of the cord with unilateral section of the chain. The most conclusiveevidence in favour of a purely nervousinterpretation of the determination of ex-citement pallor in the chameleon lies in theeffects of spinal transection with unilateralsection of the sympathetic chain (see Text-fig. 5). In this case the region posterior tothe point of section remained dark on theside on which the chain was cut, while theside on which the chain was intact exhibitedgeneralised pallor after stimulation of theroof of the mouth.

(d) Section of the chain only. Section ofthe chain only does not interfere with thegeneralised pallor which follows stimulationof the roof of the mouth. But in one ex-periment (Text-fig. 6) in which the cut went through a large ganglion in theposterior region of the trunk a narrow band of skin remained dark in thesegment of the cut.

F i g 6 Ef f e c t o f s t i m u i a t i o n o f t h e m o u t h

after section of chain involving partial de-

4. THE NATURE OF NERVOUS CONTROL OF PIGMENTARY EFFECTORACTIVITY IN THE CHAMELEON.

The evidence brought forward in section 3 clearly indicates that the pallorresulting from nocuous stimulation is predominantly, if not wholly, nervous. Twoalternatives now present themselves: is this control exercised through direct inner-vation of the pigmentary effectors, or is the production of pallor due to somechemical conditions such as oxygen-want, etc., resulting from extreme constriction(or dilation) of the peripheral arterioles? As has been pointed out by the seniorauthor, the latter possibility has too frequently been overlooked. In this case itcan be rejected on experimental grounds by cutting out the influence of the circu-lation. If the body is divided into strips by section at right angles to the caudalcephalad axis, pallor can always be induced in isolated segments of the trunk by


electrical stimulation of the caudal end of the c.N.s. The experience can more-over be repeated again and again on the same strip. It is very difficult to believethat a reversible effect of this kind could be repeated after the circulation hadbeen stopped, unless the pigmentary effector organs were in direct connectionwith the central nervous system. And it thus seems legitimate to draw the con-clusion that the production of excitement pallor depends on the direct innervationof the melanophores in Reptiles. It should be borne in mind that there is noavailable histological evidence for the direct innervation of the pigmentary effectorsin Reptiles and Amphibia, though the nervous connections of the Melanophoresof Fishes have been observed by more than one investigator.

STinULftT/onr OFr-JOVTH

TAIL

FORE UnasHIND LIMBS

Fig. 7. Diagrammatic representation of the nerve paths involved in the control of the pigmentaryeffector system of the chameleon on the basis of the experiments recorded in the text.

For the purpose of diagrammatisation the number of ganglia is reduced, and the ascending anddescending afferent paths from cloaca and mouth respectively are represented in each case bya single neurone. Section of the cord alone anterior to A restricts the pallor following stimulationof the mouth to the region in front of the cut. After section of the cord alone at the levelindicated by B, generalised pallor involving the hind extremities with the exception of the tip of thetail follows stimulation of the roof of the mouth.

With this direct evidence available, it is possible to map the innervation of thepigmentary effector system in the chameleon on the basis of the experiments re-corded in the preceding section. All the results therein recorded are representeddiagrammatically in Text-fig. 7. The experiments show that the efferent impulsesare distributed by the sympathetic nervous system, and from the fact that stimu-lation of the cloaca after transection of the cord at any level results in pallorposterior to the cut only, it may be concluded that post-ganglionic neuronessupplying each segment of the trunk are connected by pre-ganglionic neurones tothe cord in the same segment; and further, that there are no ascending nerve pathsin the chain. The same assumption explains why the section of the chain alonedoes not prevent the production of pallor behind the cut, when the mouth isstimulated. The segmental character of the response to stimulation of the mouthafter section of the cord in front of the tenth vertebra indicates that up to this


level there are no descending pre-ganglionic neurones in the sympathetic chain;while the fact that generalised pallor follows after the stimulation of the mouthwhen the cord alone is cut behind the nth-i2th vertebra, indicates that therepass into the chain about the level of the ioth-i2th vertebra some pre-ganglionicneurones which have a descending course, distributing impulses leaving the cordat this point to segments posterior to it. The fact that the tip of the tail remainsdark after section of the cord in the region behind the nth-i2th vertebra isexplicable on the assumption that these descending neurones terminate in front ofthe region where the post-ganglionic neurones supplying the pigmentary effectororgans at the extremity of the tail arise. The same assumptions account for theeffects of unilateral section of the chain at the same level as the cord and of sectionof the chain at a lower level than that at which the cord is cut in the same region.

5. ACTION OF ADRENALINE.

Some further light is thrown on the possibility that adrenaline secretion enterssignificantly into the phenomenon of excitement pallor in the chameleon by astudy of the minimal effective dose. In the first two experiments synthetic adrenaline(suprarenin hydrochloride synth. Hoechst) was employed. In all cases the injectionwas intraperitoneal to ensure rapid absorption, and the dosage indicates theequivalent amount in 1 c.c. of saline solution.

Experiment I. Two medium sized chameleons were injected with 1 : 10,000.General pallor resulted within five minutes. The pallor was more intense, and thehue more yellowish than in any case of pallor which supervened after nocuousstimulation. The condition persisted for about 36 hours.

A chameleon of the same size (as nearly as possible) became completely palewithin ten minutes after an injection of 1 : 50,000. The condition persisted forthe period of three hours during which it was under observation. The pallor inthis case was accompanied by the same intensely yellow tint. Three other chame-leons of about the same size received respectively 1 : 150,000, 1 : 300,000 and1 : 500,000. An incomplete and somewhat patchy pallor in no case extending overthe whole body ensued in all three cases, and subsided within an hour. Fourchameleons which received a dosage of 1 : 1,000,000, 1 : 5,000,000, 1 : 10,000,000,1 : 50,000,000 displayed no response whatever.

Experiment II. A second series of chameleons were injected via the peritoneumwith 1 c.c. in each case of synthetic adrenaline in the following dilutions 1 : 25,000,1 : 50,000, 1 : 100,000, 1 :200,000, 1 : 400,000,1 : 800,000,1 : 1,000,000. Those

that received 1 : 100,000 or the stronger solutions displayed complete pallor after15 minutes. The pallor was still maximal after two hours. When examined tenhours later, the pallor had not completely disappeared. Those that received dosesof 1 : 200,000 and 1 : 400,000 did not display complete pallor, but gave a noticeablereaction persisting at least two hours. With solutions of 1 : 800,000 and 1 :1,000,000the results were entirely negative.

Experiment III. Seven chameleons were injected (intraperitoneal) each with


1 c.c. of the following solutions of Parke Davis' adrenaline 1 :100,000,1 :150,000,1 : 200,000, 1 : 300,000, 1 : 400,000, 1 : 500,000, 1 : 600,000, After 15 minutes

the first three were incompletely pale. The last showed no reaction. A slightpallor was displayed by the remainder. After 30 minutes, Nos. 1, 2, 3 werecompletely pale, Nos. 4, 5, 6 still displayed a slight degree of pallor, and No. 7 adoubtful reaction. After 1 \ hours, Nos. 1,2,3 showed complete pallor, Nos. 4,5,6slight pallor. After ten hours, Nos. 1,2,3 were still noticeably paler than the controlsand No. 4 showed a slight pallor. Nos. 5, 6, 7 were all completely dark.

From these observations it is seen that there is a fairly definite thresholdeffective dose for the adrenaline pallor reaction, that the minimal quantity is froma physiological standpoint very considerable, and that this quantity produces aresponse which persists for a period which is very protracted in comparison withthe effects observed in connection with the phenomenon of excitement pallor. Itis clear that neither of these conclusions reinforce the likelihood that adrenalsecretion plays any significant part in the production of excitement pallor.

The persistence of the response to adrenaline secretion is in keeping with theeffects of adrenaline on other effector systems in cold-blooded animals. Thus thepressor effect which in mammals is so characteristically evanescent may be pro-longed over a period of over a quarter of an hour in the case of the tortoise (Hogbenand Schlapp, 1924). Whatever explanation may be given to this persistence, itimplies that if adrenal secretion entered into the phenomenon of excitement pallor,prolonged nocuous stimulation should evoke a more lasting pallor than stimulationfor shorter periods. Whether such is actually the case may be inferred from thefollowing protocol:

Timem. s.00 00 Stimulated i minute. Complete pallor at end.04 00 Dark again.05 00 Stimulated 5 minutes. Complete pallor.12 30 Dark again.13 00 Stimulated 15 minutes: darkening began about 8 minutes after stimulation

began and pallor complete by time current was cut off.

Thus in the case of a stimulus applied for one minute recovery took three minutes.With a stimulus of five minutes duration, recovery took place in two and a halfminutes, and with more prolonged stimulation recovery supervened before thecessation of stimulation.

Effects of Epinephrectomy. The effects of removal of the adrenals were investi-gated on a few acute preparations only. Had the results been otherwise the con-clusion drawn from them would be of doubtful value unless confirmed by experi-ment on chronic preparations. Unfortunately, although the adrenals are in aneminently accessible situation for operative procedure, the chameleon did not proveto be a very viable animal for laboratory purposes, and preparations that apparentlyrecovered well immediately after the operation did not survive till the wound washealed. However the application of nocuous stimuli to the mouth in animals inwhich the cord was intact or cut at the level of the thirteenth to fourteenth vertebra

3o6 LANCELOT T. HOGBEN and Louis MIRVISH

produced generalised pallor, except in cases where the chain which lies very closeto the adrenals was cut or damaged in the course of removing the latter. It wasthus possible to find no evidence of participation of increased adrenal secretionin the phenomenon of excitement pallor in the chameleon.

6. THE ACTION OF CERTAIN DRUGS.

Far too much importance has been attached by comparative physiologists tothe action of drugs, as indicative of nervous control of effector structures, appa-rently under the misapprehension in some cases that drug action is a good dealmore specific than is actually the case. It cannot be too strongly emphasised thatmost of the well-known drugs have a general protoplasmic action, and that theirselective action is generally speaking quantitative rather than qualitative. Con-clusions based on this line of evidence as brought forward in a recent paper byCarter (1925) on the supposed innervation of the cilia of veliger larvae are of littlesignificance, if unsupported by more direct forms of experimental procedure. Inthe experiments which have been conducted by us an attempt was therefore madeto discriminate between peripheral and central action. The results are essentiallywhat would be expected from the direct evidence that the melanophores of thechameleon are under c.N.s. control, tending to keep them in the contracted state.The following experiments show that strychnine and caffeine act through theC.N.S. by augmentation of the activity of the chromatic centre, and that the equallycharacteristic action of curare and atropine in the opposite sense is due to paralysisof nerve endings. Taken in conjunction with the foregoing experiments, the newdata reinforce conclusions already inferred from an independent source.

A few data regarding the action of drugs on the pigmentary effector system ofReptiles are available from the work of previous authors, who did not attempt toidentify the seat of action, except in the case of curare, where the darkening pre-viously observed by Bert to follow injection was shown conclusively by Krukenbergto be due to peripheral action on the nerve endings. Pallor following the injectionof strychnine is recorded by Briicke, Krukenberg and Keller, and an identicalresponse to caffeine is recorded by Krukenberg, who also observed darkening afteradministration of atropine.

Action of Strychnine. Conclusive proof that the pallor which accompanies theexcited state following strychnine injection acts through the c.N.s. is afforded byexperiments of which the following protocol is typical:

Timem. s.00 00 Injection of 0-0003 &&- strychnine, dark animal.00 10 Excitable but still completely dark.00 15 Pleurothotonos: pallor commences to develop.00 20 Pallor complete.00 25 Cord and chain cut at 13th vertebra.00 27 Pallor in front of cut complete: region behind noticeably darker.00 35 Pallor in front of cut complete: region behind completely dark.00 50 Ditto. Still breathing.


Action of Cocaine. The action of cocaine at first raised the hope of discriminatingbetween the alternative hypotheses to account for the light reaction adumbratedin § 2. A dose of o-ooi gm. injected into the cord in the tail region producedpallor which in a few seconds manifested itself near the seat of injection andgradually crept forwards till maximal pallor of that intense yellowish tint so charac-teristic of the response to adrenaline was general within 3—5 minutes later. Sincecocaine acts specially on the sensory nerve endings, and since the propagation ofthe effect suggested a nervous route, the inference at first appeared to justify theview that light inhibits the chromatic centre reflexly through photo-receptors inthe skin (cf. § 2 above). This conclusion did not prove to be sustained by furtherexamination, as is shown by the following facts:

(1) Injection of cocaine into a chronic preparation with section of cord andchain at about the level of the thirteenth vertebra produced generalised pallor.

(2) In chameleons injected with cocaine subsequent section of the cord andchain did not affect the generalised character of the response, nor did completedestruction of the cord in the posterior part of the trunk region.

It is thus clear that the cocaine pallor is not due simply to interference withafferent paths, since it still occurs when all efferent routes are destroyed.

Action of Histamine. Histamine has a very similar action to that of adrenalineand cocaine. An injection of o*ooi gm. (intraperitoneal) produces complete general-ised pallor within ten minutes accompanied by twitchings with complete recoverywithin 12 hours. The influence of histamine is not upon the chromatic centresince section of the cord and chain does not affect the character of the response asis the case with strychnine.

Action of Caffeine. Injection of 1 c.c. 1 per cent, solution of caffeine producedpallor within ten minutes as would be expected. After section of the cord andchain at about the thirteenth vertebra injection of the same quality produced palloranterior to the point of section. The following experiment illustrates both thewell-known action of caffeine on the higher centres of the c.N.s. and the de-cussation of fibres below the chromatic centre. The right side of the brain of achameleon was destroyed after section of the cord about the level of the ninthvertebra. Injection of the minimal effective dose of caffeine then produced palloron the right side of the body anterior to the point of section of the cord.

Action of Curare and Atropine. An injection of 1 c.c. 1 per cent, curare in palechameleons kept in darkness produces melanophore expansion with general mortu-paralysis. The same quantity of atropine produced darkening without paralysis.Stimulation of the cord after darkening produced by atropine and curare did notresult in pallor over the region innervated by the part of the c.N.s. stimulated.

7. CONCLUSIONS.

The main outcome of this investigation may be stated as follows:(1) The pigmentary effector system of the chameleon is directly innervated

through the C.N.S., and the production of excitement pallor is determined throughthis agency.


(2) The light and heat reactions are not wholly independent of the C.N.S.

A chromatic centre in the brain at least plays a static role in tending to keep themelanophores in a state of contraction, except when released from that conditionby external stimuli.

(3) While a number of circumstances make it unlikely that adrenaline playsany part in the production of excitement pallor, no evidence could be foundto support the view that excitement pallor is conditioned or reinforced by activeliberation of adrenaline into the circulation.

The authors are unable to offer any explanation of the difference between theirown findings and those of Redfield. It seems unlikely, in view of the strikinguniformity of the characteristic features of Reptilian pigmentary effector activity,that different mechanisms are involved in the production of excitement pallor inthe chameleon and in Phrynosoma. Until the influence of adrenal secretion hasbeen demonstrated in material more favourable for the study of colour responsethan in Phrynosoma, it is legitimate to express the doubt that the phenomena ofcolour response in Reptiles provides conclusive evidence of the possibility ofdefining conditions in which the liberation of adrenaline into the vertebrate circu-lation in increased quantity takes place.

8. REFERENCES.

BERT (1875). "Sur le me"canisme et les causes des changements de couleur chez le cameleon."C.R. hebdomad, de UAcad. sc. 81 .

BRUCKE (1851—2). "Ueber den Farbenwechsel der Chameleon." Sitsber. math.-naturwiss. Klasseder Kaiserl. Akad. ssu Wien, 7.

(1852). " Untersuchungen iiber den Farbenwechsel der afrikanischen Chameleon." Denkschr.der Kaiserl. Akad. Wiss. Wien, 4.

CARLTON (1904). "The colour changes of the skin of the so-called Florida chameleon." Proc.Amer. Acad. Arts Sci. 39.

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the pigmentary effector system v. the nervous … · o 09 50 pallor complete, o 11 00 noticeably...

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